Gamma-ray bursts

The nature of gamma-ray bursts are a long-standing question in high
energy astrophysics. Observations using ASCA have helped identify a
soft gamma-ray repeater and may have found counterparts to two
classical gamma-ray bursts.

Soft gamma-ray repeaters are
distinguished from classical gamma-ray bursts by their short
durations, softer gamma-ray spectra, and recurrent outbursts.
The soft gamma-ray repeater SGR1806-20 was found to be active
in September-October 1993 by BATSE. ASCA detected a burst of
photons on October 9 at the same time as BATSE registered a short
(~ 28 msec) burst. The burst photons came
from the same position as a persistent source, AX1805.7-2025,
discovered in the same ASCA observation. This persistent source is
identified with a compact radio nebula in the supernova remnant
G10.0-0.3. This compact nebula, whose radio spectrum suggests
a synchrotron origin, is most likely a plerion, a centrally
brightened pulsar-powered synchrotron nebula (Kulkarni et al.
1994 Nature 368, 129). AX1805.7-2025 is compact (less than 30''
in diameter) with a power-law X-ray spectrum, and its persistent X-ray
flux is constant on timescales of minutes to a week (Sonobe et al.
1994 ApJ 436, L23); consistent with the characteristics of isolated
pulsars. So, it can be concluded that SGR1806-20 is the persistent
X-ray and radio source, AX1805.7-2025, and indeed a neutron star.

Two classical gamma-ray bursts with positions determined using
interplanetary networks now have possible X-ray counterparts detected
using ASCA and ROSAT. The proposed identifications of GRB 781119
(Hurley etal 1996 preprint) and GRB 920501 (Murakami et al. 1996
PASJ 48, L9) are with sources having emission out to at least 8 keV
with strong absorption, consistent with Galactic column. Thus, if
these are counterparts to the gamma-ray bursts then they must
lie at least in the outer parts of the Galaxy. Further deep
observations of interplanetary network error boxes may yield more
counterparts. The hard X-ray spectra of those detected so far
show that ASCA is the best available mission to do these searches.